1. Field of the Invention
The invention is generally related to the area of optical communications. More particularly, the invention is related to optical wavelength multiplexing and add/drop devices with high reflection channel isolation and the method for making the same.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, many separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
Optical add/drop and multiplexer/demultiplexer devices are those elements often used in optical systems and networks. For example, an exchanging of data signals involves the exchanging of matching wavelengths from two different sources within an optical network. In other words, an add/drop device can be advantageously used for the multi-channel signal for dropping a wavelength while simultaneously adding a channel with a matching wavelength at the same network node. Likewise, for transmission through a single fiber, a plurality of channel signals are combined via a multiplexer to be a multiplexed signal that eventually separated or demultiplexed via a demultiplexer.
A fundamental element in add/drop devices and multiplexer/demultiplexer is what is called a three-port device. As the name suggests, a three-port device has three ports, each for a multi-channel signal, a dropped or added signal or a multi-channel signal without the dropped or added signal. FIG. 1A shows a typical design of a three-port add/drop device 100. The optical device 100 includes a common (C) port 102, a reflection (R) port 104, and a transmission (T) port 106. When the device 100 is used as a multiplexer (i.e., to add a signal at a selected wavelength λK to other signals at wavelengths other than the selected wavelength λK), the T-port 106 receives a light beam at the selected wavelength λK that is to be multiplexed into a group of beams at wavelengths λ1, λ2, . . . λN excluding the selected wavelength λK coupled in from the C-port 102. The R-port 104 subsequently produces a multiplexed signal including all wavelengths λ1, λ2, . . . λK, . . . λN.
Likewise, when the optical device 100 is used to demultiplex signals, the C-port 102 receives a group of signals with wavelengths λ1, λ2, . . . λK, . . . λN. The T-port 106 produces a signal with the selected wavelength λK while the R-port 104 subsequently produces a group of signals including all wavelengths λ1, λ2, . . . λN except for the selected wavelength λx. In general, the optical paths towards a R-port and a T-port are respectively referred to as R-channel and T-channel.
In most occasions, however, each of the ports may reflect a certain level of signal, referred to as a noise. This back reflection signal or noise can cause signal fluctuation. Thus an in-line isolator is usually inserted to eliminate this noise. FIG. 2A shows that an isolator is inserted before a group of signals with wavelengths λ1, λ2, . . . λK, . . . λN is connected to a signal C-port of a demultiplexer while FIG. 2B shows that an isolator is disposed after a group of signals with wavelengths λ1, λ2, . . . λK, . . . λN is multiplexed via a multiplexer. In either case, it is noticed that the isolator is a separate device. The combination of the isolator and a multiplexer or a demultiplexer causes some problems. One of the noticeable problems is a relatively large space required to accommodate these two individual parts. Another one is the complicated procedures involved to optically align these two parts, thus incurring additional labels and costs in real installations.
Accordingly, there is a great need for techniques for providing high isolation for the optical devices such that the errors or reflection noises are minimized, and at the same time. The devices so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process.